Preparation method of different crystal size zsm-5 molecular sieve and application thereof

Abstract

The application discloses a preparation method of ZSM-5 molecular sieves with different crystal sizes and application thereof. The method comprises the following steps: mixing a template solution and a silicon source solution, stirring for 20-40 minutes to obtain solution A; adding an aluminum source and an additive into the solution A, and stirring for 2-6 hours to obtain a gel; hydrothermally crystallizing the obtained gel precursor to obtain a crystallization liquid; centrifugally separating the crystallization liquid, and then roasting the obtained solid for 6 hours to obtain Na-type ZSM-5; and immersing the Na-type ZSM-5 into an NH4Cl solution to obtain H-type ZSM-5. The method is simple and easy to implement, and the conditions are easy to control. The obtained molecular sieve is used in the preparation of olefins in an alcohol dehydration reaction, and the ethanol conversion rate reaches 94.0%, and the ethylene selectivity reaches 85.1%.

Description

Technical Field

[0001] This invention belongs to the field of molecular sieve catalysis and alcohol conversion technology, specifically relating to a method for preparing ZSM-5 molecular sieves of different crystal sizes and their application in ethanol dehydration reaction at high space velocity. Background Technology

[0002] Ethanol dehydration to ethylene is a crucial reaction in the high-value utilization of bioethanol and the conversion of endothermic fuels. With the development of renewable carbon resource utilization and low-carbon fuel technologies, higher demands are placed on the performance of ethanol dehydration catalysts under conditions of higher throughput, higher ethylene selectivity, and long-term stable operation. Especially under high mass space velocity (MHS) conditions, the residence time of reactants in the catalyst bed is significantly shortened, leading to problems such as decreased ethanol conversion, reduced ethylene selectivity, and insufficient stability in continuous reactions.

[0003] ZSM-5 molecular sieves possess a regular MFI topology, high hydrothermal stability, and tunable acid properties, showing promising application prospects in the dehydration of ethanol to ethylene. For ZSM-5 molecular sieves, their catalytic performance depends not only on the framework acidity but also on crystal size, diffusion path length, pore accessibility, and acid site accessibility. Conventional commercial ZSM-5 molecular sieves typically have large crystal sizes and wide particle size distributions, which can easily lead to diffusion restriction under high space velocities, hindering further dehydration of ethanol to ethylene. Furthermore, excessively large crystals prolong the residence time of reaction intermediates in the pores, increasing the retention and accumulation of intermediate byproducts such as diethyl ether, thereby reducing ethylene selectivity.

[0004] In research on the synergistic control of crystal growth by adjusting the amount of template agent, the content of alkali metal oxides and crystal growth regulators, existing additives can be roughly divided into: (1) traditional organic structure directing agents or template agents, such as TPAOH, TPABr, etc., which mainly determine the formation of MFI framework and the number of nuclei; (2) surfactants or soft templates, such as CTAB, P123, etc., which are mainly used to form hierarchical pores or mesoporous structures, and may also induce ZSM-5 / ZSM-11 phase transition in some systems; (3) polymers or polyhydroxy molecules, such as PVP, starch, cellulose derivatives, etc., which mainly regulate morphology by adsorbing onto the crystal surface; by simply changing the amount of template agent, long-chain surfactants are used to construct mesopores to achieve the regulation of the number of nuclei, crystallization rate, crystal size and particle size distribution of ZSM-5 molecular sieves; for example, Li Xiangping et al. in the literature "Catalytic Performance of Submicro ZSM-5 in Ethanol Dehydration to Ethylene at High Reaction Pressure" (Energy and Environment A method for using submicron ZSM-5 molecular sieves for ethanol dehydration to ethylene is disclosed in Focus, 2014, 3(3): 292-296, DOI: 10.1166 / eef.2014.1100. This paper uses tetraethyl silicate, sodium aluminate, and tetrapropylammonium hydroxide as raw materials to synthesize submicron ZSM-5 molecular sieves via a hydrothermal method, and compares them with commercial micron-sized NKC-03A catalysts for ethanol dehydration to ethylene. CN86101615A discloses an NKC-03A ethanol dehydration catalyst, and CN101138735B discloses a catalyst for the preparation of bioethylene and its molding method. The above technologies mainly use HZSM-5 molecular sieves as the active component and are molded by adding binders or additives for ethanol dehydration to ethylene reactions. The optimal reaction space velocity is typically about 1–2 h⁻¹. -1 However, existing methods mainly focus on the formation of molecular sieve MFI framework, mesoporous construction, or morphology control, which has limitations such as insufficient precision in crystal size control, complex preparation process, low optimal reaction space velocity, and insufficient ethanol conversion and ethylene selectivity at high space velocities. Summary of the Invention

[0005] This invention aims to address the limitations of current technologies by providing a method for preparing ZSM-5 molecular sieves of different crystal sizes and their applications. The method employs a hydrothermal approach, adjusting the amount of template agent and introducing an amino acid-based crystal growth regulator with bifunctional amino and carboxyl groups during the aluminum source addition stage. This regulator can interact with the silicon-aluminum precursor in an amphoteric / carboxylate form in an alkaline synthesis solution. The synthesis process is also adjusted to control the crystal growth behavior and final crystal size of the ZSM-5 molecular sieve. ZSM-5 molecular sieves are prepared via hydrothermal crystallization. The resulting sample is then washed, dried, calcined, and subjected to ion exchange to obtain H-type ZSM-5 molecular sieves. These molecular sieves are then used in high-space velocity ethanol dehydration reactions to improve ethanol conversion and ethylene selectivity.

[0006] The technical solution of this invention is as follows: A method for preparing ZSM-5 molecular sieves with different crystal sizes, the method comprising the following steps: (1) Mix the template agent solution and the silicon source solution and stir for 20-40 minutes to obtain solution A; The mass ratio of silicon source to template agent is 0.1-1.5, more preferably 0.2-1.5.

[0007] Each 30-40 g of solution A contains 1-10 g of silicon source; (2) Add aluminum source and additive to solution A and stir for 2-6 hours to obtain gel; For every 5 g of silicon source, 0.1-0.8 g of aluminum source and 0-0.6 g of additives are added; when the amount of material added is 0, it means that no substance is added. (3) The obtained gel precursor was placed in a hydrothermal crystallization environment at 80~180 ℃ for 12-96 h to obtain a crystallization solution; (4) The crystallization liquid is separated by centrifugation, and then the obtained solid is heated to 350-550 °C and calcined for 2-6 h to obtain Na-type ZSM-5, that is, ZSM-5 molecular sieves with different crystal sizes; The following steps are also included: the Na-type ZSM-5 obtained in step (4) is immersed in NH4Cl solution and refluxed at 50-75 ℃ for 2-4 h. After reflux, the solid product is separated by vacuum filtration and dried at 80-120 ℃ for 2-4 h. The "immersion-reflux-vacuum filtration-drying" cycle is performed 1-3 times to obtain H-type ZSM-5. For every 20 ml of NH4Cl solution, 0.5–2.0 g of Na-type ZSM-5 is added; the concentration of the NH4Cl solution is 0.5–2 mol / L. The template agent in step (1) is selected from one of tetrapropylphosphoric acid hydroxide, tetrapropylammonium bromide, ethanol, and ethylenediamine, more preferably tetrapropylammonium bromide.

[0008] The silicon source in step (1) is selected from sodium silicate, silica, tetraethyl orthosilicate, tetramethyl orthosilicate, and Ludox AS-30, and more preferably sodium silicate.

[0009] The aluminum source in step (2) is selected from aluminum isopropoxide, aluminum sulfate octadecyl water, sodium aluminate, and aluminum hydroxide, and more preferably aluminum isopropoxide.

[0010] The additive mentioned in step (2) is selected from one or more of hexadecyltrimethylammonium bromide, glycine, lysine, and β-alanine. More preferably, the additive is glycine.

[0011] The crystallization time in step (3) is preferably 24-72 h.

[0012] In step (4), the centrifugation speed is 8000-12000 rpm, more preferably 11000-12000 rpm.

[0013] The heating rate in step (4) is 1~5 ℃ / min; the calcination time is preferably 4-6 h.

[0014] The ZSM-5 molecular sieve described above has a typical MFI framework structure and a crystal size of 100~3800 nm.

[0015] The ZSM-5 molecular sieves of different crystal sizes prepared by the method are used to prepare olefins in alcohol dehydration reactions.

[0016] Specifically, the steps include: passing alcohols into a fixed-bed reactor loaded with ZSM-5 molecular sieves of different crystal sizes, and incubating at 250~300 ℃ for 2-100 h. -1 The reaction proceeds to yield an olefin; The alcohols are selected from one or more of ethanol, methanol, propanol, and polyols; The preferred airspeed is 4-70 h. -1 .

[0017] The essential features of this invention are: This invention selects small-molecule glycine as an additive in the TPABr / sodium silicate / low silica-alumina ratio synthesis system. Glycine molecules are small in size and possess both -NH2 and -COOH properties. In alkaline synthesis solutions, they can form a bifunctional -NH2 / -COO- state, enabling coordination, hydrogen bonding, and electrostatic interactions with Al species, Na+, silanol groups, and silica-alumina precursors. This regulates the polymerization, nucleation, and crystal growth rates of silica-alumina species in the gel, thereby obtaining ZSM-5 molecular sieves with a more concentrated crystal size distribution while maintaining the MFI framework structure, and altering the polymerization and nucleation growth behavior of silica-alumina species. Therefore, this effect differs from the hydrophobic chain / micelle effect of long-chain quaternary ammonium salt soft templates such as CTAB, and also from changes in the number of nuclei resulting from simply altering the TPABr content.

[0018] In this invention, an amino acid-based crystal growth regulator is introduced during the aluminum source addition stage: first, the template agent is mixed with the silicon source to allow the silicon source to fully hydrolyze and form a silicon-containing precursor; then, a glycine-based crystal growth regulator is added simultaneously with the aluminum source, allowing glycine to preferentially participate in the complexation / assembly process of aluminum species and early silicon-aluminum precursors. If glycine is added before silicon source hydrolysis or after crystallization, it is difficult to effectively control the nucleation of silicon-aluminum species and early crystal growth.

[0019] This invention shortens the diffusion path of ethanol, diethyl ether intermediates, and generated water inside and outside the molecular sieve crystal by adjusting the crystal size of ZSM-5 to about 300 nm, improves the accessibility of acid sites on the outer surface and near the pores, reduces diffusion restriction and diethyl ether retention under high space velocity conditions, thereby promoting further dehydration of ethanol to ethylene and improving ethanol conversion, ethylene selectivity, and continuous reaction stability.

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention synthesizes ZSM-5 molecular sieves with different crystal sizes, exhibiting a gradient range of 100–3800 nm, by adjusting the amount of template agent and adding crystal growth regulators. This provides an effective method for the orderly synthesis of ZSM-5 molecular sieves with varying crystal sizes. Compared to currently available commercial catalysts, we achieve a more stable crystal size distribution, and the synthesized catalyst crystals are fixed within a specific numerical range.

[0021] 2. The ZSM-5 molecular sieve with a crystal size of 300 nm prepared in this invention exhibits optimal catalytic performance in the high-space velocity (WHSV) ethanol dehydration reaction, showing high ethanol conversion and ethylene selectivity. By adjusting the molecular sieve synthesis method, small-crystal-size ZSM-5 was synthesized at 275 °C and WHSV = 70.2 h. -1Under the same conditions, its ethanol conversion rate reached 94.0% and ethylene selectivity reached 85.1%, which is significantly better than the 88.4% ethanol conversion rate and 62.7% ethylene selectivity of commercial ZSM-5 molecular sieve under the same conditions. 3. The ZSM-5 molecular sieve with a crystal size of about 300 nm prepared by this invention maintains a high ethanol conversion rate and ethylene selectivity throughout the 25 h continuous reaction, with a small attenuation range. The ethanol conversion rate decreases by 1.7% and the ethylene selectivity decreases by 6.9%, showing better stability. It has good application potential in high space velocity ethanol dehydration systems. 4. The method of synthesizing ZSM-5 molecular sieves of different crystal sizes in this invention is simple and easy to implement, and the conditions are easy to control. It can stably synthesize molecular sieves of specific crystal sizes. Attached Figure Description

[0022] Figure 1 XRD (X-ray diffraction) analysis of samples 1-4 in Examples; Figure 2 Here are scanning electron microscope (SEM) images of Comparative Sample 1 and Examples 1-4; Figure 3 The graph shows the relationship between space velocity and ethanol conversion rate when comparative sample 1 and examples 1-4 molecular sieve catalysts are used in the ethanol dehydration reaction. Figure 4 The graph shows the relationship between space velocity and ethylene selectivity when comparative sample 1 and examples 1-4 molecular sieve catalysts are used in the ethanol dehydration reaction. Figure 5 The graph shows the relationship between space velocity and diethyl ether selectivity when comparative sample 1 and examples 1-4 are used as molecular sieve catalysts in the ethanol dehydration reaction. Figure 6 The graph shows the 25-hour stability test results for samples from Comparative Example 1, Example 1, and Example 2. Detailed Implementation

[0023] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. It should be noted that the following embodiments are illustrative and not limiting, and should not be construed as limiting the scope of protection of the present invention. All raw materials used in the following embodiments and comparative examples are commercially available.

[0024] Example 1: ZSM-5 molecular sieve was synthesized using a hydrothermal crystallization method. First, 12.0 g of tetrapropylammonium bromide solution was weighed and mixed with 26.0 g of deionized water, and stirred for 30 min to obtain a template agent solution. Then, 5.1 g of anhydrous sodium silicate was weighed and dissolved in 10 mL of deionized water to obtain a silicon source solution. Next, the silicon source solution was added dropwise to the template agent solution at a rate of 2–3 s / drop. After complete addition, stirring continued for 20 min until complete hydrolysis was achieved, yielding solution A. Then, 0.68 g of aluminum isopropoxide was dissolved in 5 mL of water and added dropwise to solution A, with stirring continued for 2 h to form a homogeneous sol. After stirring, the sol was transferred to a hydrothermal reactor and sealed. The reactor was then placed in a forced-air drying oven and crystallized at 170 ℃ for 3 days. After crystallization, the sol was washed with deionized water by centrifugation (10000 rpm) until neutral and dried at 80 ℃. Finally, the sample was calcined at 550 °C for 6 h to obtain Na-type ZSM-5 molecular sieve with a crystal size of 300 nm.

[0025] The calcined Na-type ZSM-5 sample (1 g) was added to a 1 mol / L ammonium chloride solution (20 mL) and refluxed at 80 °C for 5 h. After cooling, it was centrifuged and washed, and then dried at 120 °C for 4 h. The above ion exchange steps were repeated twice. Subsequently, the sample was calcined again, heated from 30 °C to 550 °C and held at that temperature for 4 h, and then cooled. The heating and cooling rates were both 5 °C / min, yielding the H-type ZSM-5 molecular sieve.

[0026] Example 2: The other steps are the same as in Example 1, but 0.4 g of glycine is added as a crystal growth regulator when adding the aluminum source. The crystallization, washing, drying, calcination and ion exchange steps are the same as in Example 1, and H-type ZSM-5 molecular sieve with a crystal size of 100 nm is obtained.

[0027] Example 3: The other steps are the same as in Example 1, but the amount of template agent is adjusted to 4 g. The remaining crystallization, washing, drying, calcination and ion exchange steps are the same as in Example 1, to obtain H-type ZSM-5 molecular sieve with a crystal size of 1200 nm.

[0028] Example 4: The other steps are the same as in Example 1, except that the silicon source is replaced with 8.35 g of Ludox AS-30 silica sol. The remaining crystallization, washing, drying, calcination and ion exchange steps are the same as in Example 1, to obtain H-type ZSM-5 molecular sieve with a crystal size of 3600 nm.

[0029] Comparative Example 1: Commercially available ZSM-5 molecular sieve with a SiO2 / Al2O3 molar ratio of 25 was selected as the comparative sample.

[0030] Figure 1 The XRD patterns of ZSM-5 molecular sieve catalysts prepared in different embodiments of the present invention are shown. Each sample exhibits distinct ZSM-5 characteristic diffraction peaks at approximately 7.9°, 8.8°, 23.1°, 23.9°, and 24.4° at 2θ, indicating that the samples synthesized using our method maintain a typical MFI framework structure. In the embodiments of the present invention, changing the molecular sieve gel composition does not affect the crystal structure of the molecular sieve, and the synthesized samples are ZSM-5 crystals.

[0031] Figure 2 The images show scanning electron microscope (SEM) images of the ZSM-5 catalysts from Examples 1-4 and Comparative Example 1. The SEM images show that we altered the crystal size of ZSM-5 by changing the composition of the molecular sieve synthesis gel. Compared to the comparative example, the crystal size distribution of the samples is more uniform and concentrated within a certain range: approximately 300 nm in Example 1, approximately 100 nm in Example 2, approximately 1200 nm in Example 3, approximately 3600 nm in the comparative example, and 2000–5000 nm in the comparative example.

[0032] Catalytic dehydration of ethanol using catalysts: The samples obtained from Example 5 and Comparative Example 1 were tableted, crushed, and sieved into 20-40 mesh catalyst particles for evaluation of the fixed-bed ethanol dehydration reaction. Nitrogen was used as the carrier gas at a flow rate of 1.5 L / h. 0.1 g of catalyst was mixed thoroughly with 1.5 g of silica sand and packed into a fixed-bed reactor. Before the reaction began, the catalyst in the fixed-bed catalyst section was heated to the reaction temperature of 275 °C for 2 h to activate the catalyst. After activation, ethanol, the reaction product, was introduced, and the reaction space velocity was adjusted from 2 to 70.2 h by controlling the ethanol pumping rate. -1 The generated product was directly introduced into a gas chromatograph for detection. The reaction system needed to be stable for 2 hours. Five data points were collected for each space velocity condition for equilibration.

[0033] Detection of catalytic ethanol dehydration products: Quantitative analysis of the dehydration products of ethanol was performed using gas chromatography. The chromatographic column was a Wax capillary column; the column temperature program was: initial temperature 50 °C, heating rate 10 °C / min, increasing to 180 °C. o C, hold for 5 min; detector is a hydrogen flame ionization detector, detector temperature 300 °C. o C; single injection 0.2 μL; carrier gas is high-purity nitrogen.

[0034] Ethanol dehydration occurs via two pathways: intermolecular dehydration, an endothermic reaction, produces diethyl ether; and intramolecular dehydration, an exothermic reaction, produces ethylene.

[0035] Figure 3 The ethanol conversion rate of the catalysts in Comparative Example 1 and Examples 1-4 under different space velocity conditions is shown.

[0036] Figure 4 The ethylene selectivity of the catalysts in Comparative Example 1 and Examples 1-4 for catalyzing the dehydration reaction of ethanol under different space velocity conditions was measured.

[0037] Figure 5 The diethyl ether selectivity of the catalysts in Comparative Example 1 and Examples 1-4 for catalyzing the dehydration reaction of ethanol under different space velocity conditions is shown.

[0038] The results showed that the ethanol conversion and ethylene selectivity of all samples decreased with increasing space velocity (WHSV). Under high space velocity conditions, the catalytic performance of each catalyst followed the order: Example 1 > Example 2 > Example 3 > Example 4 > Comparative Example 1. Example 1 consistently exhibited the highest ethanol conversion and ethylene selectivity at WHSV = 70.2 h⁻¹. -1 Under the same conditions, the ethanol conversion rate reached 94.0%, and the ethylene selectivity reached 85.1%; while in Comparative Example 1, the ethanol conversion rate decreased to 88.4%, and the ethylene selectivity decreased to 62.7%. These results indicate that appropriately reducing the crystal size can effectively shorten the diffusion path of molecules in the pores, improve the accessibility of acidic sites, and weaken the influence of diffusion limitation on the reaction process. However, when the crystal size was further reduced to 100 nm, its catalytic performance did not continue to be superior to the 300 nm sample, indicating that a smaller crystal size is not always better.

[0039] The results of ethanol dehydration catalyzed by ZSM-5 molecular sieve catalysts of different crystal sizes in this invention are shown in Table 1. These include reaction mass hourly space velocity, ethanol conversion, ethylene selectivity (intramolecular dehydration), and diethyl ether selectivity (intermolecular dehydration). The reaction conditions were 1 atm and 275 °C. Table 1 Comparison of catalytic performance results

[0040] Stability test: Figure 6 For 275 °C and WHSV=70.2 h -1Under the specified conditions, the stability of the reaction of Examples 1, 2, and Comparative Example 1 was tested over a continuous 25-hour period. The results showed that the performance indicators of all three samples decreased with increasing reaction time, but significant differences in stability existed among the samples. Example 1 consistently maintained the highest ethanol conversion rate and ethylene selectivity throughout the test, with the smallest decrease; Example 2 was second; and Comparative Example 1 showed the lowest and most significant decrease. These results indicate that the ZSM-5 molecular sieve with a crystal size of 300 nm achieves a good balance between diffusion performance, framework stability, and resistance to deactivation.

[0041] The results from the examples show that, under essentially the same conditions, Example 1, without the addition of glycine, yielded crystals of approximately 300 nm; Example 2, with the addition of 0.4 g of glycine along with the aluminum source, yielded crystals of approximately 100 nm; Example 3, by reducing the amount of template agent, yielded crystals of approximately 1200 nm; and Example 4, by changing the silicon source, yielded crystals of approximately 3600 nm. These results demonstrate that the present invention achieves crystal size gradient through a combination of control over "template agent dosage—silicon source type—amino acid additive—addition sequence," rather than simply screening a single parameter.

[0042] The glycine additives in this invention differ from traditional quaternary ammonium salt template agents or long-chain surfactants. In the synthesis solution, they primarily regulate crystal growth rather than simply acting as structure guides or mesoporous templates. Their amino and carboxyl groups can interact at multiple points with aluminum species, sodium ions, and the aluminum-silicon precursor, altering the local coordination environment and polymerization rate of aluminum-silicon species in the precursor gel, thereby regulating the nucleation density and crystal growth rate of the ZSM-5 molecular sieve.

[0043] The approximately 300 nm ZSM-5 crystal prepared in this invention maintains the MFI framework and a sufficient number of acid sites while significantly shortening the diffusion path of molecules within the crystal and improving the accessibility of acid sites on the outer surface and at the pore openings. This allows ethanol and diethyl ether intermediates to more fully contact the effective acid sites and further convert to ethylene within a short residence time. Simultaneously, the shortened diffusion path also facilitates the timely removal of products and generated water from the pore opening region, reducing the competitive adsorption and coverage of acid sites by water molecules and byproducts. Therefore, this structure can support high ethanol conversion, ethylene selectivity, and stability at high mass hourly space velocities (MHSVs).

[0044] It is important to note that the experimental results of this invention also show that smaller crystal size is not always better. Although the 100 nm sample has a shorter diffusion path, the increased proportion of acid sites on the outer surface, crystal defects, or changes in water adsorption state may result in lower ether selectivity and stability compared to the 300 nm sample. Therefore, a crystal size of approximately 300 nm represents a comprehensive balance between diffusion performance, the number of effective acid sites, framework integrity, and resistance to deactivation. This can serve as a creative argument distinguishing it from "simple nano-sizing."

[0045] The present invention has been described above by way of example. It should be noted that any simple modifications, alterations or other equivalent substitutions that can be made by those skilled in the art without creative effort without departing from the core of the present invention fall within the protection scope of the present invention.

[0046] Matters not covered in this invention are common knowledge.

Claims

1. A method for preparing ZSM-5 molecular sieves with different crystal sizes, characterized in that, The method includes the following steps: (1) Mix the template agent solution and the silicon source solution and stir for 20-40 minutes to obtain solution A; The mass ratio of silicon source to template agent is 0.1-1.

5. (2) Add aluminum source and additives to solution A and stir for 2-6 hours to obtain gel; For every 5.0 g of silicon source, 0.1~0.8 g of aluminum source and 0~0.6 g of additives are added. The additives mentioned are hexadecyltrimethylammonium bromide, glycine, lysine, or β-alanine; (3) The obtained gel precursor was placed in a hydrothermal crystallization environment at 80~180 °C for 12-96 h to obtain a crystallization solution; (4) The crystallization liquid is separated by centrifugation, and then the obtained solid is heated to 350-550 °C and calcined for 2-6 h to obtain Na-type ZSM-5, that is, ZSM-5 molecular sieves with different crystal sizes.

2. The method for preparing ZSM-5 molecular sieves of different crystal sizes as described in claim 1, characterized in that, The process also includes the following steps: immersing the Na-type ZSM-5 obtained in step (4) in NH4Cl solution, refluxing at 50-75 ℃ for 2-4 h, filtering after reflux to obtain solid product, and drying at 80-120 ℃ for 2-4 h; performing 1-3 cycles of "immersion-reflux-filtration-drying" to obtain H-type ZSM-5; For every 20 ml of NH4Cl solution, 0.5–2.0 g of Na-type ZSM-5 is added; the concentration of the NH4Cl solution is 0.5–2 mol / L.

3. The method for preparing ZSM-5 molecular sieves of different crystal sizes as described in claim 1, characterized in that, The template agent in step (1) is selected from one of tetrapropylphosphoric acid hydroxide, tetrapropylammonium bromide, ethanol, and ethylenediamine. The silicon source in step (1) is selected from sodium silicate, silica, tetraethyl orthosilicate, tetramethyl orthosilicate, or Ludox AS-30. The aluminum source in step (2) is selected from aluminum isopropoxide, aluminum sulfate octadecyl water, sodium aluminate or aluminum hydroxide.

4. The method for preparing ZSM-5 molecular sieves of different crystal sizes as described in claim 1, characterized in that, In step (1), the mass ratio of silicon source to template agent is 0.2-1.5; The crystallization time in step (3) is 24-72 h; In step (4), the centrifugation speed is 8000-12000 rpm; The heating rate in step (4) is 1~5 ℃ / min; the calcination time is 4-6 h.

5. The method for preparing ZSM-5 molecular sieves of different crystal sizes as described in claim 1, characterized in that, The template agent in step (1) is tetrapropylammonium bromide; the silicon source is sodium silicate; The aluminum source in step (2) is aluminum isopropoxide, and the additive is glycine.

6. The method for preparing ZSM-5 molecular sieves of different crystal sizes as described in claim 2, characterized in that, The H-type ZSM-5 molecular sieve has an MFI framework structure and a crystal size of 100~3800 nm.

7. The application of ZSM-5 molecular sieves of different crystal sizes prepared by the method described in claim 1 or 2, characterized in that, Used in the preparation of olefins from alcohols during dehydration reactions.

8. The application as described in claim 7, characterized in that, comprising the step of passing an alcohol into a fixed bed reactor loaded with ZSM-5 molecular sieves of different crystal size at a temperature of 250-300 °C for 2-100 h -1 the following reaction to obtain an olefin; The alcohols are selected from one or more of ethanol, methanol, propanol, and polyols.

9. The application as described in claim 8, characterized in that, Airspeed is 4-70 h -1 .

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